In the past, numerous incinerators have been devised in order to burn airborne organic matter entrained in air so as to convert the matter into essentially carbon dioxide and water vapor. Such incinerators have usually employed a preheater in the form of a heat exchanger provided with a plurality of tubes, through which the air and combustible matter passes, as these tubes are heated by the products of combustion being exhausted from the combustion chamber.
In the present day, more and more emphasis is being placed upon thoroughly incinerating all organic volatiles and other organic matter entrained in air, particularly the volatile organic compounds (VOC's) in air discharged from industrial plants. Such volatiles emanate from paint booths, drying ovens, and the like. In the past, such incinerators have usually been quite complex structures.
The concentration of VOC's contained in the atmosphere expelled from certain industrial processes, such as the exhaust from ovens used for curing various types of coatings, is usually much below a concentration required for producing a combustible mixture or even for the lower explosion limit (LEL).
Therefore, these VOC's have to be oxidized to harmless gases by incineration. They cannot be converted to products of combustion simply by being burned because of their low concentration in the mixture. However, when these compounds are exposed to an increased temperature for sufficient time in the presence of oxygen, they can be oxidized into harmless products of combustion. This process is usually accomplished at temperatures of between 1250.degree. F. and 1500.degree. F. and with dwell times for the organic matter from 0.2 seconds to 1 second.
In many applications where incineration of the VOC'S are required, the temperature of the mixture containing the VOC's is relatively low, 250.degree. F. to 350.degree. F. compared to the temperature required f or oxidation of the VOC's. Therefore, in order to conserve energy, most incinerators are of the recuperative type which allow for pre-heating the incoming mixture containing VOC's by the hot gases generated from the incineration process. Therefore, the energy added to accomplish the incineration can be greatly reduced if the incoming mixture can be separated from the combustion gases and pre-heated normally to about 1000.degree. F. If pre-heating can be accomplished to this temperature, then the energy added to the mixture to be incinerated is only that required to heat the mixture from 1000.degree. F. to the incineration temperature, which is usually in the range of 1250.degree. F. to 1500.degree. F.
In order to prewheat the incoming mixture containing the VOC's, some type of air-to-air heat exchanger is required. A typical heat exchanger used for this application would employ tubular heat transfer surfaces which could be constructed in many geometries. Most practical heat exchangers of this type use either cylindrical or rectangular tubular heat transfer surfaces.
In conventional incinerators employing pre-heating, it would not be desirable to mix the incoming polluted air mixture with the relatively clean air from the incineration process. Therefore, a gas tight seal is required to prevent the intermixing of the two. This gas tight seal is usually produced by welding the heat transfer tubular surfaces into headers. In many designs, the tube headers, in which the heat transfer tubes are welded, are exposed to the high exhaust gas temperatures of the incinerator. Failure of these welds and the heat exchangers associated with these types of incinerators have been a common problem in the past. Also, the failure of these heat exchangers has created expensive repairs.
A problem associated with the type of heat exchangers described above is that the resulting structure must provide for the expansion and contraction of the multiple tubes contained within one heat exchanger when different tubes are exposed to different temperatures. Carbide precipitation within the weld is another problem in prior art devices and in many instances, the weldment is subjected to high stresses as a result of the expansion and contraction of the heat transfer tubes.
Attempts have been made, in the past, to only weld one end of each heat transfer tube into one of the tube plates and allow the other end to float in the other tube plate. While this concept does provide for expansion and contraction of the tubes, it adds another design problem of providing a means to seal each tube while it expands and contracts within the tube plate. Thus, these types of incinerators have not been satisfactory or widely accepted because of leakage at the seal.
The present invention overcomes the difficulties prescribed above by providing an inexpensive and yet quite durable and efficient incinerator.
I have found that, to achieve virtually 100% destruction of VOC's, it is not necessary to provide a specific dwell time for the air and VOC mixture in the combustion chamber at a constant, specific, elevated temperature. In other words, the mixture of incoming air and VOC's does not have to be held at a specified constant temperature before it enters the primary heat exchanger. The rate at which the VOC's are oxidized depends upon, among other things, the temperature of the mixture. Thus, the dwell time required will depend upon the temperature versus time relationship experienced by the mixture as it passes through the combustion chamber.
In the preferred embodiment of this invention, the heat transfer to the incoming mixture occurs throughout the incineration chamber. This concept provides for more efficient heating of the incoming mixture while, at the same time, insuring that the time-temperature relationship, for essentially 100% destruction of the VOC's is maintained.